The level of the electric output signal of optical-to-electric transducers for optical receivers should lie within a given range. The output level is correlated with the intensity of the optical input signal. This intensity, however, depends on the transducer's field of application and exhibits temperature-, time-of-day-, system-modification-, etc. dependent variations. This necessitates an automatic level adaptation in the optical-to-electric transducer. The further this level adaptation goes, the greater the so-called dynamic range of the optical-to-electric transducer will be.
In conventional systems, this level adaptation is carried out exclusively in the electric portion. This has turned out to be very costly, particularly if the following requirements are to be fulfilled to a high degree:
Large bandwidth, PA1 great dynamic range, PA1 high sensitivity, and PA1 immunity to overloading. PA1 bandwidth in the Gb/s range, PA1 dynamic range of approx. 80 dB, PA1 insensitivity of, e.g., -32 dBm at 5 Gb/s. PA1 immunity to overloading guaranteed.
A transducer wherein an incoming optical signal passes through an optical amplifier before entering the transducer proper is known from "Electronic Letters", Sep. 13th 1990, Vol. 26, No. 19, pages 604-605. Due to a fiber preamplifier, the sensitivity of this transducer is considerably better than that of: conventional transducers. However, no measures to increase the dynamic range are mentioned there.